For those that don't have specialist software (and/or don't fancy freeware/shareware/pirated software), but only a Windows install, the following is the best you can do (which isn't half bad by the way) with standard Windows tools. If your picture is anything other than a JPG format (JPG and GIF is best for internet anyway) this method won't work to best effect, thus I include instructions on how to best convert them using standard tools as the first step.
1. To change your picture format from any other popular format (read BMP, GIF, PNG, TIFF) to JPG just right-click on the picture in Windows Explorer --> Open With --> Windows Picture and Fax Viewer. Then click on the Save icon at the bottom, select JPEG in the drop-down list and click on the Save button. This method will result in the best quality JPG picture (with standard tools) when converted from another format.
2. To resize just right-click on the (new JPEG version of) the picture in Windows Explorer --> Open With --> Microsoft Office Picture Manager. Then go to Edit Pictures --> Resize, and select the resolution you want. Please note that you can also do things like Crop, adjust Brightness and Contrast, and do Red Eye Removal in this program. This method will result in you keeping your original picture as is (which is often important) and you being able to post up better edited pictures.
3. Now, the final step (which is often overlooked) is good image compression. This is very important when posting on the web as a better compressed picture is smaller in storage size and thus loads faster. I've found the best standard tool for this to be Paint (yes Paint), although you don't want to use Paint as an image format convertor, as it doesn't retain the image quality too well when converting. So now you just right-click on the (new JPEG version of) the picture in Windows Explorer --> Open With --> Paint. Then simply click on File --> Save, and then close Paint. This method will result in you getting the best compression without losing and quality or clarity in your picture.
Happy converting/resizing your piccies!